Fix PrivateRefCount hash table key size

Hi hackers,

While working on [1], I noticed that there is a type mismatch when computing
the key size for the PrivateRefCount hash table. Indeed, the first
PrivateRefCountEntry member type is Buffer (int), so the keysize should be
sizeof(Buffer) and not sizeof(int32).

PFA attached a patch to fix it.

It has been kind of automatically detected while working on [1], so I'm pretty
confident there are no other type mismatches for hash table key size.

Note that this is exactly the kind of issue that the macro proposed in [1] would
avoid.

[1]: https://www.postgresql.org/message-id/flat/aS2b3LoUypW1/Gdz%40ip-10-97-1-34.eu-west-3.compute.internal

Regards,

-- 
Bertrand Drouvot
PostgreSQL Contributors Team
RDS Open Source Databases
Amazon Web Services: https://aws.amazon.com